Disk drive capacity has been increasing by reducing the spacing between the tracks. Radial motion that does not follow a repeating pattern is known as a non-repetitive run out. A servo system needs to move the head instantaneously to stay on-track and avoid read/write errors even for non-repetitive run out.
Servo systems may tend to amplify relative radial motion between the head and data tracks at higher frequencies. This relative radial motion may be further amplified by air circulation within the drive, also known as windage. Windage may amplify the radial motion, and may be further increased in systems with more than one disk.